Effects Of Modulating Activation Frequency And Velocity Of Stretch-shortening Contractions In Muscles Of Mice

Daily activity involves frequent stretch-shortening contractions (SSCs). Basic animal research on SSCs has tested a limited number of contraction velocities and activation frequencies. Snell dwarf mice display an extended lifespan but compromised health span in the form of muscular weakness. This makes Snell dwarf mice potentially useful for gaining insight on aspects of muscle contraction in relation to human aging and occupational health. PURPOSE: To determine whether exposure to SSCs (2 sessions per week for 1 month) of varying velocities and activation frequencies would elicit distinct changes in muscle mass and performance in wild-type and Snell dwarf mice. METHODS: In each session, 3-month-old mice were anesthetized and needle electrodes were placed subcutaneously. Plantarflexor muscles (N = 5-8 per group) were exposed to 8 sets of 10 SSCs at 30°/s (rotational velocity) at 150 Hz (activation frequency), 60°/s at 150 Hz, or 60°/s at 60 Hz. RESULTS: For wild-type mice, exposures of 60°/s at 150 Hz and 60°/s at 60 Hz induced muscle mass gains normalized to tibial length (8.9 ± 0.4 v. 8.6 ± 0.5 mg/mm and 8.9 ± 0.6 v. 8.6 ± 0.4 mg/mm, respectively, p < 0.05), whereas exposure of 30°/s at 150 Hz did not (9.1 ± 0.3 v. 9.0 ± 0.6 mg/mm, p = 0.7349). Exposure of 60°/s at 150 Hz and 30°/s at 150 Hz induced greater final SSC peak torques (normalized to body weight) relative to 60°/s at 60 Hz (0.56 ± 0.11 and 0.48 ± 0.09 v. 0.38 ± 0.09 mN-m/g, respectively, p < 0.05). Muscle mass gain was concomitant with high total work, and performance gain with optimal activation frequency. For Snell dwarf mice, all exposures induced comparable muscle mass loss with no change in peak torque. CONCLUSIONS: Muscle mass and performance were differentially dependent on SSC activation frequency and velocity in wild-type animals, where Snell dwarf mice uniformly displayed no performance gains. These outcomes and parameters should be considered in research design on SSCs. Further investigation of SSCs could provide insight on ways to improve muscle performance and health span in relation to lifespan extension. Disclaimer: "The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention.” This study was supported by internal National Institute for Occupational Safety and Health funds.